Conventionally, when a multilayer coating film is formed by a coating method in which two or more kinds of coating materials are applied using a wet-on-wet technique and then subjected to a curing treatment, thermosetting coating materials for forming layers constituting the multilayer coating film have been selected so that all the layers can be cured at the same heating temperature after all the coating materials are applied, and then the multilayer coating film as a whole has been cured. In this case, there has been a problem that the obtained multilayer coating film is inferior in surface texture and gloss to that obtained by baking a lower layer and then applying and baking coating materials for forming an uppermost layer. In this connection, various methods have been proposed to improve the surface texture and the gloss of a multilayer coating film.
For example, Japanese Unexamined Patent Application Publication No. Hei 10-277478 (Document 1) discloses a method for forming a coating film in which a color base coating material, a base coating material containing an effect pigment (a metal pigment and/or a luster pigment) and a clear coating material are sequentially applied using a wet-on-wet technique, and then a baking treatment is performed to cure each layer. In this method, the coating materials are adjusted so that the viscosity-increase starting times for the color base coating material, the base coating material containing an effect pigment (a metal pigment and/or a luster pigment), and the clear coating material would be longer in this order. Thus, before the viscosity of the clear coating material for forming the uppermost layer increases along with the curing, the color base coating material and the base coating material containing an effect pigment (a metal pigment and/or a luster pigment) for forming the lower layers start to be cured.
Japanese Unexamined Patent Application Publication No. 2002-35679 (Document 2) discloses a method for forming a coating film in which an intermediate coating material, a base coating material and a clear coating material are sequentially applied to an electrodeposition-coated substrate, and then the three layers are simultaneously cured by baking. In this method, an excellent finished appearance can be achieved by using the coating materials satisfying the conditions that when measured using each of coating materials which contain 90% by mass of non-volatile content, the minimum viscosities with respect to temperature satisfy the relationship: the intermediate coating material ≧ the base coating material ≧ the clear coating material; and the curing start temperatures satisfy the relationship: the intermediate coating material ≦ the base coating material ≦ the clear coating material.
Japanese Unexamined Patent Application Publication No. 2005-177680 (Document 3) discloses a coating method in which, when an intermediate coating material and a top coating material are applied using a wet-on-wet technique and then simultaneously baked, the intermediate coating film is cured prior to the curing of the top coating film by taking an advantage of the difference in curing rate. By this coating method, the image clarity can be reliably ensured.
Japanese Unexamined Patent Application Publication No. 2005-193107 (Document 4) discloses a coating method in which an intermediate coating material and a top coating material are applied using a wet-on-wet technique and then simultaneously cured. Here, a reduction in the smoothness of the top coating film attributable to the mixing between the intermediate coating film layer and the top coating film layer is considered as the cause of inferior surface texture and gloss of the multilayer coating film. Hence, in this method, in order to prevent the mixing, after the intermediate coating material is applied, a curing catalyst for accelerating the curing of the intermediate coating film layer is applied to an uncured surface of the intermediate coating film layer.
Meanwhile, it is known that the cause of reduced surface texture and gloss of a multilayer coating film is a residual of a solvent in the multilayer coating film prior to curing by baking. Particularly, if the solvent in the multilayer coating film is rapidly evaporated during a curing reaction, the surface of the multilayer coating film is roughened. In order to prevent this, the following methods have been proposed. For example, Japanese Unexamined Patent Application Publication No. 2000-84463 (Document 5) discloses a method for forming a coating film based on two-stage heating, the method comprising the steps of: applying a thermosetting coating material solution; performing heating at a low temperature; and performing heating at a high temperature. Moreover, Japanese Unexamined Patent Application Publication No. 2004-275966 (Document 6) discloses a method for forming a coating film comprising the steps of: sequentially applying an intermediate coating material, a base coating material and a clear coating material using a wet-on-wet technique; and performing a heat treatment in two stages including a low-temperature heating stage and a high-temperature heating stage. In these methods, a solvent in a coating material is gradually evaporated without curing the coating material at the low-temperature heating stage; thereafter, a thermosetting resin contained in the coating material of each layer is cured at the high-temperature heating stage.
As described above, various methods have been proposed heretofore to improve the surface texture and gloss of a multilayer coating film. However, coated articles more excellent in appearance quality are demanded as to, for example, steel plates for automobile, and the like. Accordingly, a further improvement in a wet-on-wet coating method is ever desired.